In some electronic ballast circuits used to drive gas discharge lamps, a rectifier converts AC power to DC power. A two transistor inverter then converts the DC power to AC power at a very high frequency, usually on the order of 30 KHz. The AC power is coupled to the lamps, usually via a transformer. A feedback circuit uses the transformer to synchronize the opening and closing of the transistors. One such circuit for driving an array of gas discharge lamps is shown in U.S. Pat. No. 5,124,619.
Such an arrangement is efficient. However, the circuit is susceptible to "cross conduction". If both transistors are on at the same time, a direct short across the output of the rectifier results. This, in turn, results in component failure.
Lamp failure presents an opportunity for cross-conduction. When a gas discharge lamp begins to fail, it operates likes a diode. The result is that the current though the lamp has a "clipped" sinusoid waveform. The clipped sinusoid waveform effects the current in the transformer, thus impacting the feedback circuit that controls the inverter transistors. Absent a protection circuit, cross conduction results, and inverter components may fail.